exhaust leak

I always use a large wide file for exhaust surfaces like that. But you have to do it correctly. The file must be held flat against the surface so that the whole perimeter is filed at the same time, this way it will stay flat. You cannot push harder against one side or the other, it takes a little practice but will work like a charm if done correctly!!
The alternate method is to remove the manifold and sand it against some coarse sand paper. The sand paper must be taped (or held) against a hard flat surface. Then you simply rub the surface against the paper....BUT AGAIN, you must not tilt the manifold as you stroke it against the sand paper. You have to keep the surface dead flat against the sand paper so as not to create a taper or uneven surface.
The file method is easier and more fool proof.